1. Field of the Invention
The invention relates to a bicycle wheel rim, more particularly to an airtight bicycle wheel rim.
2. Description of the Related Art
A conventional bicycle wheel rim is generally formed from an elongated metal strip by bending the strip into an annular shape and subsequently connecting two opposite ends of the strip. Referring to FIGS. 1 and 2, a conventional bicycle wheel rim 1 is shown to include spaced left and right tire retaining walls 12 and a spoke mounting wall 11 interconnecting inner edges of the tire retaining walls 12. The spoke mounting wall 11 cooperates with the tire retaining walls 12 to form a tire receiving space 4 thereamong, and is formed with a plurality of spoke fastening holes 10 that are slightly staggered and that are arranged generally along a central portion of the spoke mounting wall 11 for mounting of spoke fasteners 2 thereon. Each spoke fastener 2 has an internally threaded shank portion 20 for connecting threadedly with one end of a corresponding spoke 31. As shown in FIG. 3, the spokes 31 mount the bicycle wheel rim 1 on two opposite ends 30 of a hub 3.
Some of the drawbacks of the conventional bicycle wheel rim 1 are as follows:
1. Because the spoke fastening holes 10 are formed in the central portion of the spoke mounting wall 11, air in the tire receiving space 4 can easily leak out of the wheel rim 1 via the spoke fastening holes 10. PA1 2. To prevent undesired deflation due to the presence of the spoke fastening holes 10 in the central portion of the spoke mounting wall 11, a two-layer tire that includes an inner tire 41 and an outer tire 42 is employed. Air is confined within the inner tire 41, whereas the outer tire 42 engages the tire retaining walls 12 so as to retain the inner tire 41 in the tire receiving space 4. Extra costs are thus incurred due to the use of the two-layer tire. PA1 3. The head portion 21 of each spoke fastener 20 extends into the tire receiving space 4. As such, an additional lining 43 is needed to prevent damage to the inner tire 41 due to direct contact between the spoke fasteners 20 and the inner tire 41.